Magnar Hernes

Mechanical analysis of press-pack IGBTs

Abstract At present two packages for IGBT devices are available for applications in the MW power range: the bonded power module and the press-pack housing. Power modules have been object of extensive research including their thermo-mechanical characterisation under variable operating conditions and the analysis of their failure mechanisms. These aspects have been critical to improve the manufacturing process, increase reliability and provide lifetime estimations. The press-pack package eliminates bonding wires and solder layers, and is claimed to offer improved power cycling lifetime. However, the knowledge on press-pack devices is much less mature with only limited data published in literature related to their thermo-mechanical behaviour. This paper presents results of FEM simulations on a full 3D model of a press pack IGBT under power-cycling conditions and during the clamping process.

Pressure Tolerant Power Electronics for Deep and Ultra-Deep Water

Summary This paper presents results from an ongoing research project (SINTEF 2011) on pressure-tolerant power electronics. The main goal of the research is to provide and demonstrate solutions that enable power-electronic converters to operate in pressurized environments. Oil companies have plans for subsea processing of oil and gas. Today’s concept considers power-electronic converters in 1-bar vessels. As the depth and the converter power rating increase, these vessels become increasingly bulky and heavy. Pressurebalanced converters would allow lower vessel-wall thickness, thereby giving lower weight, and simpler cooling owing to improved heat conduction through the vessel walls. The new concept considers power-electronic converters placed in vessels completely filled with appropriate dielectric liquid able to operate at pressures from 1 bar up to several hundred bar. Dielectric fluids are required to prove their properties, especially those related to insulation, incompressibility, and heat transport. The methodology presented here considers a mechanical adaptation of components followed by various laboratory experiments for verifying or correcting the proposed solutions (Pittini et al. 2010; Hernes and Pittini 2009; Petterteig et al. 2009). Standard off-the-shelf components and special pressure-adapted components have been subjected to various provocative pressurization tests up to 300 bar. Tests clarified the need for special adaptation of some components, while others could be used without any modification. Subsequently, a full converter phase leg has been built, submerged in dielectric liquid, and tested in full operation up to 300 bar. This phase leg is based on a press-pack insulated gate bipolar transistor (IGBT) modified for operation at high pressure. Measurements performed at different pressures demonstrate that there is no relevant difference in terms of electrical parameters between this modified IGBT and a standard IGBT. Long-term tests proved the concept’s validity. The next step will be to test bonded IGBT technology in a high-pressure environment, and finally, we will realize a demo converter to be located at a suitable subsea site.

Possible failure modes in Press-Pack IGBTs

Abstract Reliability of Press-Pack IGBTs is a topic with limited published data and information. This paper presents results of a power cycling test with state-of-the-art high power devices. An accelerated lifetime test scheme was defined, and six out of eight devices were tested until failure. A microscopy analysis has been performed on some of the failed devices, and they have all failed in a very similar manner. In order to gain additional information about the thermal–mechanical stress, a detailed 3D Finite Element Method (FEM)-analysis has been conducted. The combined results from power cycling, microscopy and FEM have been concluded to two possible failure modes in Press-Pack IGBTs: Gate oxide damage and micro arcing.

Determination of the thermal and electrical contact resistance in press-pack IGBTs

Press-pack IGBTs are manufactured with a multi-layered structure composed of internal layers with different geometrical shape and physical properties. The device is clamped and an external pressure is applied on the housing in order to establish a sufficient electrical and thermal contact between these layers. The modeling of interlayer contact resistances is necessary to characterize the behaviour of the device in Finite Element Methods (FEM) simulations. However, this modeling task can be difficult since they are pressure sensitive and cannot be easily determined by a direct measurement. This paper presents a method to identify these contact resistances combining FEM analyses with experimental measurements only on the external surfaces of the chip assembly. Indeed, the method procedes with an iterative process where contact resistances are changed according to a genetic search algorithm in order to match the results of the FEM simulations with the experimental measurements.

Liquid insulation of IGBT modules: Long term chemical compatibility and high voltage endurance testing

To facilitate operation of power electronics for subsea operation at ambient pressure components have to be submerged in a liquid. Equipment and schemes for testing of long term properties have been developed. Several techniques were used to investigate compatibility between a silicone gel and various insulation fluids. Equipment for electric endurance testing of power electronic components at dc stress under controlled temperature and humidity was developed. Performance of high voltage diodes covered with various insulation liquids, coatings and gel covering were studied.

Analysis of power cycling for semiconductor devices in modular multilevel converters

This paper presents a numerical analysis of the temperature profiles within the semiconductor devices of modular multilevel converters (MMCs). These temperature profiles are essential for assessing the power-cycling lifetime of IGBT modules, which influences the converter reliability. An electro-thermal simulation model has been established, combining a dynamic thermal model of the semiconductor devices with a detailed model of the MMC topology and its associated control loops. Conduction losses and switching losses in the semiconductors are calculated online in the simulation, using the instantaneous voltages and currents in the devices. The presented simulation results reveal low losses and limited thermal stresses in terms of peak-to-peak temperature variations in the devices, for both a 200-level MMC operating under staircase modulation, as well as for a 20-level MMC operating with pulsewidth modulation (PWM). This indicates that the application of conventional IGBT modules in high power MMCs is mainly constrained by the current switching capability and not by the power-cycling lifetime.

Improving the short circuit ruggedness of IGBTs

Abstract The demands on reliable and fault tolerant power electronic devices are increasing. One opportunity to increase the IGBT short circuit ruggedness is to modify the thermal capacitance and the thermal resistance close to the chip and hence extend the possible short circuit duration. Therefore simulations with different metallization, bond wire and chip interconnect materials are compared to identify the most promising solution for enhancing short circuit capability of IGBTs.